Plural expansion-compression refrigeration cycle with a fractionating column



Aug. 22, 1967 R; E. M HARG PLURAL EXPANSION-COMPRESSION REFRIGERATION CYCLE WITH A FRACTIONATING C OLUMN -27 E/hane Filed Dec. 5, 1964 Low Pressure Load mfermed/a/e v Pressure L000 High Pressure Load n s a I V g i K w. \l n 1* D V "3 "3 f if e Compressor wm Rece/ver /w 5 p I in f 5 v c v L "3 Q e v S S e E f ffhy/ene Fracfionaf/on r4 TTOR/YEYS United States Patent assignor to Des Plaines, 111., a

This invention relates to an improved fractionating column and more specifically relates to a more efiicient utilization of the heating and cooling energy necessary to operate a fractionating column. Still more specifically, this invention relates to the integration of a refrigeration unit with a processing unit having a low temperature fractionator wherein high pressure refrigerant vapor is condensed in the reboiler heat exchanger of the low temperature fractionator and a portion of the condensed refrigerant is used to cool the overhead vapors of said low temperature fractionator.

There are a number of examples of processes in which low temperatures are employed requiring the use of a refrigeration system having One or more fractionating columns with subambient reboiler temperatures. An outstanding example is an ethylene separation process to produce high purity ethylene from a gaseous mixture containing ethylene. Although there are many variations, most ethylene units have an ethylene fractionating column wherein it is desired to separate ethylene from ethane. Generally in the past these columns have been operated at about a reflux temperature of 60 F. and a bottom temperature of about -20 F. at a pressure of about 150 p.s.i.a. Typically these fractionators have operated on a heat pump arrangement (the overhead vapors are compressed in a separate compressor condensed by the reboiler of the fractionator and then collected as a liquid in a high pressure reflux drum from which liquid reflux is returned to the top of the fractionating column). These conditions make it necessary to utilize two columns in series in order to have sufiicient trays to achieve the desired ethylene-ethane separation and since the top temperatures are so low, the top column must be manufactured of nickel alloy steel. Generally these two columns are placed in an upright position side by side and the overhead vapors from the bottom column are introduced into the bottom part of the overhead column while the liquid from the bottom part of the overhead column is introduced into the top of the bottom column. The low temperatures required for the reflux liquid requires the use of ethylene refrigerant or an equivalent low temperature refrigerant in order to obtain the required top temperature.

It is an object of this invention to integrate a refrigeration system comprising a compressor and a circulating refrigerant with a low temperature fractionating column such that at least a portion of the high pressure refrigerant vapor is condensed in the reboiler heat exchanger of said fractionating column.

It is another object of this invention to modify a heat pump fractionator in a process having a refrigeration system such that at least a portion of the high pressure refrigerant vapor is condensed in the reboiler of said fractionator and at least a portion of the condensed refrigerant is utilized to cool the overhead vapors from said fractionator.

It is a more specific object of this invention to provide a more economical ethylene fractionator in an ethylene separation process by integration of the ethylene process with a refrigeration system such that at least a portion of the circulating refrigerant is condensed in the ethylene fractionator reboiler.

3,33%,76l Patented Aug. 22, 1967 These and other objects will become more apparent in the light of the following detailed description.

The present invention obviates some of the economical disadvantages of the prior methods especially in an ethylene fractionating column in an ethylene seapration process as hereinbefore described. By utilizing the process improvement of the present invention only one column is needed, it can be made from ordinary carbon steel instead of alloy steel, there is an overall reduction in required horsepower, and the compressor on the overhead vapors of the heat pump fractionator can be eliminated. This is accomplished by maintaining the operating pressure on the ethylene column from about p.s.i.a. to about 400 p.s.i.a. and preferably about 280 p.s.i.a. and bottom temperatures of from about 20 F. to about +60 F. At said preferable pressure, a reflux temperature of about -20 F. and a bottom temperature of about +20 F. is required to separate ethylene from a mixture of ethane and ethylene. The 20 F. top temperature is suitably served by a propylene refrigeration system and the +20 F. reboiler temperature provides a sink for propylene refrigeration compression heat. The propylene refrigeration system comprises a compressor and a circulating propylene refrigerant. Low pressure propylene vapor is compressed in said compressor and the resulting high pressure propylene vapors are condensed. The condensed propylene is introduced into the refrigerant side of a cooling load such as a heat exchanger wherein the propylene is vaporized by maintaining the cooling side of the heat exchanger at said low pressure. The vaporization of propylene requires the absorption of heat which is extracted from the process side of the load thereby cooling the process material. The low pressure propylene vapor is returned to the compressor to repeat the cycle. In many cases the high pressure propylene vapor can be condensed by cooling water or the like. In the present process where in there is at least one fractionating column whose reboiler temperature is sufficiently low, at least a portion of the high pressure refrigerant vapor is condensed in the reboiler. Specifically, in an ethylene separation process having an ethylene fractionating column whose pressure is about 280 p.s.i.a., whose top temperature is about -20 F. and Whose reboiler temperature is about +20 F., the propylene refrigerant is introduced into the heating side of the ethylene column reboiler and condensed therein and at least a portion of the condensed propylene is introduced into the ethylene column overhead cooler to condense the overhead vapors from the fractionator. Thus, the fractionator is operated on a modified heat pump basis except the heating and cooling of the fractionator material is accomplished by an external circulating refrigerant instead of the fractionator material itself.

Although the improvement of this invention is applicable to any process having a refrigeration system and a low temperature fractionating column reboiler temperature (subambient), it is especially suitable for a process to separate substantially pure ethylene from mixture of normally gaseous hydrocarbons. Typical sources of such mixtures are petroleum refinery gases. For example, in a fluid catalytic process employed to convert heavy hydrocarbons into high octane gasoline, after separating the normally liquid hydrocarbons from the total reactor efliuent, a normally gaseous product stream is produced containing nitrogen, carbon monoxide, hydrogen, paraflin hydrocarbons containing up to 5 carobn atoms per molecule, olefin hydrocarbons containing up to 5 carbon atoms per molecule and trace quantities of triple-bonded hydrocarbon molecules. Another source of said mixtures comprises heater efiiuent from steam pyrolysis processing units utilized to produce light olefins. This ethylene process application of the improvement of this invention is further illustrated by reference to the accompanying figure.

A feed mixture containing ethylene flows through flow conduit 1 and into depropanizing step 2 to remove components having at least 4 carbon atoms per molecule. This depropanizing step is carried out in a fractionating column having typical operating conditions of 250 p.s.i.a., a top temperature of from about +10 F. to about -l F. and a bottom temperature of about 225 F. The net overhead material including the ethylene is recovered from depropanizing step 2 in flow conduit 3 where it flows into treating step 4.

The extent and amount of treating of depropanized feed is dependent upon the source of the gaseous feed mixture and the types of undesirable components present. It will generally be necessary to remove a substantial portion of the water from the feed mixture so as to avoid ice and hydrate formation and plugging of cold downstream equipment. Accordingly, a dryer will generally be present either in the treating section or after the compression step hereinafter described. In addition, other objectionable contaminants include sulfur compounds, acetylene, carbon dioxide, carbon monoxide and organic acids. Acetylene may be removed by means of a selective hydrogenation reactor wherein the acetylene is converted to ethylene over a suitable hydrogenation catalyst such as a chromium-cobalt-molybdenum catalyst or other wellknown hydrogenation catalysts at temperatures in the range of from 400 to 600 F. and pressures of from 100 to 400 p.s.i.a. Other impurities may be removed by gas purification treatment such as scrubbing of the feed with compounds selective for impurities such as ethanolamines, caustic wash and water wash. It is preferable to finish all the treating and compression operations thereby physically separating all the condensed water prior to the drying step in order to reduce the amount of water to be removed by the dryers. Typically, the highest pressure in the system will be in the range of from 400 to 600 p.s.i.a. with preferable pressures of from 500 to 550 p.s.i.a. Therefore the feed mixture flows through flow conduit 3, treating step 4, flow conduit 5, compression shep 6, flow conduit 7, drying step 8 and into flow conduit 9. The dryer may be a solid bed type containing alumina, silica gel, molecular sieves or other well-known dehydrating agents. It is preferable to reduce the water content of the gas to less than 10 parts per million.

The dried feed gas flows through cooling step 10 and into flow conduit 11. The cooling step comprises a series of heat exchangers utilizing propylene and ethylene refrigerants as cooling agents wherein the feed is lowered to a temperature of about -40 F. to about 100 F. The feed gas thereupon flows into absorption step 12 wherein the feed countercurrently contacts a lean oil comprising a light hydrocarbon such as propane or propylene such that said lean oil absorbs substantially all of the hydrocarbons having a higher boiling point than methane while absorbing less than substantially all of those components having a lower boiling point than ethylene. The rich oil leaves absorption step 12 through flow conduit 13 and into demethanizing step 14. The purpose of the absorption step is to reduce the operational severity of the demethanizing step although said absorption step may be omitted where the economics do not justify the use of such a step.

The demethanizing step is accomplished by a fractionating column having typical operating conditions of about 550 p.s.i.a., a top temperature of from about -100 F. to about 150 F. and a reboiler temperature of from about 60 F. to about 100 F. Top temperatures closer to l50 F. must be maintained if absorption step 12 is omitted whereas top temperatures of l00 to 120 F. are adequate with the absorption step. This latter top temperature range is attained by utilizing an ethylene refrigeration system. The demethanized mixture having ethylene as its lightest component leaves step 14 through flow conduit 15 and into deethanizing step 16.

The deethanizing step is accomplished by a fractionating column having typical operating conditions of about 350 p.s.i.a., a top temperature of about 0 F. and a bottom temperature of about F. The net overhead material from the deethanizer comprises ethane and ethylene while the net bottom material comprises propane and propylene. The net overhead from the dect'hanizer is removed through flow conduit 17 where it enters ethylene fractionating column 18.

The ethylene fractionating column is preferably operated at a pressure of about 280 p.s.i.a., wherein ethylene is separated from ethane by maintaining a top temperature of about -20 F. and a bottom temperature of about +20 F. The overhead material comprising ethylene vapor is withdrawn from fractionator 18 through flow conduit 19, through the process side of heat exchange cooler 20 wherein the ethylene is condensed, through flow conduit 21 and finally into overhead receiver 22. A liquid portion of the overhead is returned by means of flow conduit 23 to ethylene fractionator 18 as reflux. The net overhead portion is withdrawn from receiver 22 through flow conduit 24 and comprises the desired substantially pure ethylene product. The ethylene column bottom fraction is removed through flow conduit 25 where a portion of said fraction passes through flow conduit 26 through the process side of heat exchange heater 28 and returns to ethylene fractionator 18 by means of flow conduit 29. The net bottom fraction is withdrawn through flow conduit 27 and comprises ethane. The top temperature requirement of about -20 F. is suitably maintained by the use of propylene refrigerant as a cooling agent in cooler 20. The bottom temperature requirement of +20 F. allows the use of the reboiler duty and bottom material as a sink for propylene refrigeration compression heat.

The propylene refrigeration system accordingly is integrated with the heating and cooling requirements for ethylene column 18. Low pressure propylene flows through flow conduit 45 and into a multi-stage compressor 43 wherein the pressure of the propylene is increased either directly to a high pressure level or additionally to one or more intermediate pressure levels depending on the total refrigeration load. One typical embodiment comprises a three stage compressor and the propylene refrigerant is compressed to two intermediate pressure levels before reaching the high pressure level. A portion of the higher intermediate pressure propylene flows through flow conduit 30, through the heating side of heat exchange heater 28, wherein the propylene is condensed while simultaneously heating the process material flowing through the process side of heater 28, through flow conduit 31 and into surge vessel 35. The remaining portion of the higher intermediate pressure propylene flows into the third stage of the compressor where the propylene is compressed to a high pressure level, withdrawn through flow conduit 32, through cooler 44 wherein the high pressure propylene is condensed, an expansion valve 50 and finally into a high pressure cooling load. This load may comprise the heat exchangers upstream of the absorption step to cool the feed mixture as hereinbefore described, as -a cooling agent to condense the high pressure ethylene vapor in the ethylene refrigertation system and other heat exchange duties. The propylene refrigerant is withdrawn from high pressure load 33 at said higher intermediate pressure where it flows through flow conduit 34 and into surge vessel 35. A portion of the propylene flowing in flow conduit 34 is in the vapor phase and a portion is in the liquid phase. The higher intermediate pressure liquid propylene from heater 28 and load 33 are combined and are withdrawn from surge vessel '35 through flow conduit 36, passed thru an expansion valve 51 after which the liquid enters intermediate pressure load 40. The vaporized propylene leaves vessel 35 through flow conduit 47 and returns to the suction side of the third stage compressor.

The propylene refrigerant is withdrawn from intermediate pressure load 40 at a lower intermediate pressure where it flows through flow conduit 37 and into vessel 38. A portion of the propylene flowing in flow conduit 37 is in the liquid phase and a portion is in the vapor phase. Liquid propylene is Withdrawn from vessel 38 through flow conduit 39, passed thru an expansion valve 52 after which a portion is sent to low pressure load 48 by means of flow conduit 42. The remaining liquid portion flows through flow conduit 41 and into the cooling side of cooler 20. The vaporized propylene leaves vessel 38 through flow conduit 46 and returns to the suction side of the second stage compressor.

The vaporized low pressure propylene is withdrawn from cooler 20 through flow conduit 41' where it joins the vaporized low pressure propylene flowing in flow conduit 42' from low pressure load 48 and the resulting mixture flows into the suction side of the first stage compressor by means of flow conduit 45. Intermediate pressure load 40 may comprise the depropanizer overhead condenser, the deethanizer overhead condenser and other heat exchange cooling duties. Low pressure load 48 may comprise additional heat exchange cooling duties besides the ethylene column overhead cooler 20' In overall effect, fractionator 18 is operated on a modified heat pump basis with substantial savings in both equipment and operating costs. Fractionator 18 is a single column, constructed out of inexpensive carbon steel and needs no overhead compressor. The compression is accomplished by the already existing compressor 43 and amounts to an incremental loading increase on an existing compressor rather than a separate, smaller compressor requiring extra control system, plot area, maintenance and other features necessary to the installation of an extra compressor.

The following example is presented to further illustratethe process improvement of this invention but it is not intended to limit the scope of the invention to the materials or the arrangement shown.

Example I Processing equipment is arranged substantially as shown in the accompanying figure. A feed mixture is introduced in flow conduit 1 at a rate of 3502.8 moles per hour of which 1243.4 moles are ethylene. The feed mixture is subjected to the processing sequence shown in steps 2, 4, 6, 8, 10, 12, 14 and 16. The deethanized material flowing through flow conduit 17 at a total rate of 1455 moles per hour of which 1202.7 moles are ethylene is introduced into ethylene fractionator 1-8 at a temperature of F. Fractionator 18 is maintained at a top pressure of about 270 p.s.i.a. which results in a bottom pressure of about 279 p.s.i.a.

Overhead ethylene vapor is removed from fractionator 18 through flow conduit 19 at a rate of 6006 moles per hour (168,162 lbs./hr.)' at a temperature of 23 F. whereupon it flows into the process side of cooler 20. About 19,780,000 B.t.u./hr. are removed from the overhead vapor in cooler 20 which results in the condensation of 5045.5 moles per hour of overhead ethylene while leaving 961.0 moles per hour of ethylene in the vapor phase. This mixed phase ethylene, still at a temperature of 23 F., flows through flow. conduit 21 and into overhead receiver 22. Liquid ethylene reflux is removed from receiver 22 at a rate of about 4804.8 moles per hour through flow conduit 23 and returns to fractionator 18. Net ethylene overhead is withdrawn through flow conduit 24 at a rate of 1201.2 moles per hour of which about 961 mo es are in the vapor phase and 240.2 moles are in the liquid phase. 3

Bottom ethane liquid is removed from fractionator 18 through flow con-duit 25 at a temperature of +18 F. and at a rate of 1942.6 moles per hour. The net bottoms are withdrawn from the system through flow conduit 27 at a rate of 253.8 moles per hour. The remaining 1688.8 moles per hour of bottoms flows through flow conduit 26 and into reboiler heater 28 wherein it absorbs about 23,520,000 B.t.u./hr. of heat. Said remaining bottoms is returned to fractionator 18 through flow conduit 29 at a temperature of +20 F. and comprises 546.1 moles of ethane vapor and 1142.7 moles of liquid ethane.

A propylene refrigeration system is arranged substantially as shown in the accompanying figure except the compressor is a four stage compressor whose suction pressure on the first stage is 4 p.s.i.g., whose first stage discharge pressure is 15 p.s.i.g., whose second stage discharge pressure is 33 p.s.i.g., whose third stage discharge pressure is p.s.i.g., and whose fourth stage discharge pressure is 215 p.s.i.g. Propylene refrigerant vapor at a temperature of 43 F., and at a pressure of 4 p.s.i.g. is introduced into the first stage of compression at a rate of 172,012 lb./hr. and is discharged at a pressure of 15 p.s.i.g. and a temperature of +5 F. Additional propylene vapor refrigerant at a pressure of 15 p.s.i.g. is mixed with the first stage discharge and the combined vapor mixture flows into'the second stage of compression at a rate of 303,170 lbs/hr. and a temperature of 10 F. The propylene vapor refrigerant is withdrawn from the second stage at a temperature of +30 F. and a pressure of 33 p.s.i.g. where it is mixed with still additional propylene vapor refrigerant at 33 p.s.i.g. and the mixture flows into the third stage of compression at a temperature of +25 F. and at a rate of 395,645 lb./hr. The propylene vapor refrigerant is withdrawn from the third stage at a temperature of F. and a pressure of 85 p.s.i.g. A portion of the third stage discharge propylene vapor is introduced into flow conduit 30 at a rate of 130,667 lbs/hr. whereupon it is introduced into heater 28. This portion is entirely condensed at 85 p.s.i.g. and cooled to +42 F. while evolving 23,520,000 B.t.u./hr. of heat.

A portion of the circulating propylene liquid refrigerant is introduced into flow conduit 41 at a rate of 113, 78 lb./hr., at a pressure of 15 p.s.i.g. and at a temperature 22 F. Said portion passes through the refrigeration side of cooler 20 wherein the propylene is maintained at 4 p.s.i.g. thereby vaporizing the propylene. As a result of the vaporization 19,780,000 B.t.u./ hr. of heat are absorbed by the propylene and the vaporized propylene leaves cooler 20 at a rate of 113,678 lbs/hr. and at a temperature of -43 F.

I claim as my invention:

1. In a process having a refrigeration system comprising a compressor and a circulating refrigerant and a unitary fractionating tower operating at a single pressure for free flow of fluids throughout the tower and having a reboiler and overhead condenser the temperatures of which are subambient, an improvement which comprises introducing high pressure refrigerant vapor from said compressor into the reboiler of said fractionator, condensing at least a portion of the refrigerant to a liquid, expanding the condensed refrigerant to an intermediate pressure utilizing intermediate pressure refrigerant for cooling duty thereby partially vaporizing the refrigerant and leaving an intermediate pressure refrigerant liquid portion, reducing said intermediate presseure liquid portion to a low pressure and introducing said liquid into said condenser while vaporizing the refrigerant liquid in said condenser and returning the resultant low pressure refrigerant vapor to said compressor.

2. In a process having a refrigeration system comprising a compressor and a circulating refrigerant wherein cooling is accomplished by vaporization of said refrigerant, a unitary fractionating tower operating at a single pressure for free flow of fluids throughout the tower and having a reboiler heater and an overhead cooler, an improvement in the operation of said fractionating tower which comprises introducing at least a portion of high pressure refrigerant vapor from said compressor into the reboiler of said fractionating tower, condensing the refrigerant by extracting heat out of the refrigerant and into the reboiler bottoms material, expanding at least a portion of the high pressure condensed refrigerant from the reboiler to an intermediate pressure, utilizing intermcdiate pressure refrigerant for cooling duty thereby partially vaporizing the refrigerant and leaving an intermediate pressure refrigerant liquid portion, reducing said intermediate pressure liquid portion to a low pressure and introducing said liquid into said overhead cooler while vaporizing the refrigerant liquid in said overhead cooler at said low pressure whereby the overhead material from said tower is cooled by the vaporization of the latter mentioned refrigerant portion, returning the resulting low pressure refrigerant vapor to said compressor and compressing the refrigerant vapor to said high pressure.

3. The process of claim 2 further characterized in that the refrigerant is a hydrocarbon having from about 2 to 4 carbon atoms per molecule.

4. A process for the separation of ethylene from a principally gaseous mixture containing ethylene which comprises the steps:

introducing said mixture into a depropanizing fractionating column thereby substantially removing components heavier than propane from said mixture,

introducing the depropanizer overhead portion of said mixture into treating equipment to remove at least one component selected from the group consisting of acetylene, sulfur compounds, carbon oxides and water,

introducing the contaminant free mixture into a demethanizing fractionating column thereby substantially removing the methane and components having lower boiling points than methane,

introducing the bottom stream of the demethanizer into a deethanizing fractionating column and separating said bottom stream into an overhead fraction comprising ethane and ethylene and a bottoms fraction comprising propane and propylene,

introducing said overhead fraction into a unitary ethylene fractionating column operating a single pressure for free flow of fluids throughout the tower and having a reboiler heat exchanger and an overhead heat exchanger,

the heat energy to said reboiler being supplied by introducing high pressure circulating refrigeration vapor into the refrigerant side of the reboiler and condensing the refrigerant therein, expanding at least a portion of the condensed high pressure refrigerant liquid to an intermediate pressure, utilizing intermediate pressure refrigerant for cooling duty thereby partially vaporizing the refrigerant and leaving an intermediate pressure refrigerant liquid portion, reducing said intermediate pressure liquid portion to a low pressure and introducing said liquid into the refrigerant side of the overhead heat exchanger while vaporizing the refrigerant liquid in said overhead heat exchanger at said low pressure thereby extracting heat out of the substantially pure ethylene process material and vaporizing the last mentioned refrigerant portion, compressing the resultant low pressure refrigerant vapor to said high pressure and returning it to said reboiler heat exchanger,

and withdrawing a substantially pure ethylene product as a net overhead fraction from said ethylene fractionating column.

5. The process of claim 4 further characterized in that the pressure of the ethylene fractionating column is maintaincd within the range of about p.s.i.a. to about 400 p.s.i.a. and the bottom temperature is maintained from about -20 F. to about 60 F.

6. The process of claim 4 further characterized in that the refrigerant is a hydrocarbon having from about 2 to about 4 carbon atoms per molecule.

7. In a process having a refrigeration system comprising a compressor and a circulating refrigerant, and a unitary fractionator operating at a single pressure for free fiow of fiuids throughout the tower and having a reboiler heat exchanger and an overhead heat exchanger, an improvement which comprises introducing the low pressure refrigerant vapor into a compressor, passing a portion of the refrigerant vapor from said compressor at an intermediate pressure and introducing it into the refrigerant side of said reboiler heat exchanger thereby condensing it, compressing the remaining portion of the refrigerant vapor to a high pressure, condensing said high pressure vapor by contacting it with a cooler, expanding the condensed high pressure refrigerant to said intermediate pressure, utilizing intermediate pressure refrigerant for cooling duty thereby partially vaporizing said refrigerant and leaving an intermediate pressure refrigerant liquid portion, combining the first mentioned intermediate pressure condensed refrigerant with the second intermediate pressure liquid portion, expanding the resulting combined intermediate pressure refrigerant liquid to a second lower intermediate pressure, utilizing the second lower intermediate pressure refrigerant for cooling duty, thereby partially vaporizing the refrigerant and leaving a second lower intermediate pressure refrigerant liquid portion, expanding the second lower intermediate pressure refrigerant liquid to a low pressure and introducing said liquid into said overhead heat exchanger, vaporizing said low pressure refrigerant liquid in said overhead heat exchanger and returning the low pressure refrigerant vapor to said compressor.

8. The process of claim 6 further characterized in that the refrigerant comprises propylene.

References Cited UNITED STATES PATENTS 1,853,743 4/1932 Pollitzer 62-40 X 2,500,353 3/1950 Gantt 62--28 X 2,534,274 12/1950 Kniel 52- 10 X 2,573,341 10/1951 Kneil 6228 X 2,692,484 10/ 1954 Etienne 62-28 X 2,777,305 1/1957 Davison 6228 X 3,037,359 6/1962 Knapp.

3,212,276 10/1965 Eld et al. 62-28 X NORMAN YUDKOFF, Primary Examiner.

WILBUR L. BASCOMB, JR., Examiner.

V. W. PRETKA, Assistant Examiner. 

1. IN A PROCESS HAVING A REFRIGERATION SYSTEM COMPRISING A COMPRESSOR AND A CIRCULATING REFRIGERANT AND A UNITARY FRACTIONING TOWER OPERATING AT A SINGLE PRESSURE FOR FREE FLOW OF FLUIDS THROUGHOUT THE TOWER AND HAVING A REBOILER AND OVERHEAD CONDENSER THE TEMPERATURES OF WHICH ARE SUBAMBIENT, AN IMPROVEMENT WHICH COMPRISES INTORDUCING HIGH PRESSURE REFRIGERANT VAPOR FROM SAID COMPRESSOR INTO THE REBOILER OF SAID FRACTIONATOR, CONDENSING AT LEAST A PORTION OF THE REFRIGERANT TO A LIQUID, EXPANDING THE CONDENSED REFRIGERANT TO AN INTERMEDIATE PRESSURE 